1. Field of the Invention
The present invention relates generally to an active vibration damping device which is interposed between two members of a vibration system for connecting these two members or mounting one of these members on the other member in a vibration damping manner, so as to actively damp or reduce vibration transmitted from one of the two members to the other. More particularly, the present invention is concerned with a pneumatically operated active vibration damping device which includes an air chamber formed between the two members, for applying an oscillating force between the two members based on a periodic change of the air pressure in the air chamber.
2. Description of the Related Art
An active vibration damping device is known as one type of a vibration damping device, such as a vibration damping coupling (bushing) or mount, which is interposed between two members of a vibration system so as to flexibly connect these two members or mount one of these members on the other member in a vibration damping manner. Such an active vibration damping device, as disclosed in JP-A-60-8540 (publication of Japanese unexamined patent application), JP-A-61-2939 (publication of Japanese unexamined patent application), and JP-U-61-191543 (publication of Japanese unexamined utility model application), for example, includes: a first and a second mounting member which are spaced apart from each other; an elastic body elastically connecting the first and second mounting members; and an oscillating force generating device which is adapted to generate and apply an oscillating force between the two mounting members, thereby adjusting vibration damping characteristics of the vibration damping device. The active vibration damping device is operable for generating the oscillating force corresponding to the vibration to be damped, and applying the oscillating force to the vibration member whose vibration to be damped, so that the active vibration damping device is capable of eliminating or reducing the vibration to be damped by canceling the input vibration by the oscillating force, or is capable of actively adjusting spring characteristics thereof according to the input vibration so as to exhibit effectively low dynamic spring constant thereof. The thus constructed active vibration damping device is considered to apply as an engine mount or a body mount for an automotive vehicle.
There is also proposed an active vibration damping device of another type wherein an air chamber is formed between a first and a second mounting member and is alternately connected to a vacuum source and an atmosphere by a switching operation of a switch valve, causing the oscillating force based on a periodic change of the air pressure in the air chamber, at a frequency corresponding to the frequency at which the switch valve is alternately placed in two operating positions, namely a "vacuum position" for communication of the air chamber with a vacuum source, and an "atmospheric position" for communication of the air chamber with the atmosphere. This pneumatically operated type of active vibration damping device does not require a heavy and complicated member such as an electromagnetic actuator to be incorporated therein, resulting in reduction in the number of the required components, the size, weight and the required amount of electric power consumption of the device.
In the pneumatically operated type of active vibration damping device constructed as described above, it is important and desirable to control the switch valve and the negative pressure in the air chamber so that not only the frequency and phase but also the amplitude of the oscillating force correspond to those of the vibration to be damped, thereby improving the active damping effect with respect to the vibration of the vibratile member of the vibration system.
To meet the need indicated above, there is proposed to (a) detect the frequency, phase and amplitude of the vibration to be damped, by using a suitable sensor such as an accelerometer or acceleration sensor, or estimate those frequency, phase and amplitude according to predetermined data maps, (b) control the frequency and phase of the switching operation of the switching valve, so that the frequency and phase of the oscillating force correspond to those of the vibration to be damped, and (c) control the negative pressure in the air chamber so that the amplitude of the oscillating force corresponds to that of the vibration to be damped.
Where the pneumatically operated vibration damping device is used for an automotive vehicle, and the air intake system of the engine of the vehicle is used as the vacuum source, it is difficult to regulate the negative pressure in the air intake system as a vacuum source, and it is therefore difficult to obtain the oscillation force whose amplitude corresponds to that of the vibration to be damped. The active vibration damping device does not provide a sufficient active vibration damping effect, if the amplitude of the oscillating force does not accurately correspond to that of the vibration of the object.
To meet recent requirements of automotive vehicles such as an improved specific fuel consumption and purification of exhaust gases, there has been proposed an improved internal combustion engine whose combustion mode is selectable from a stoichiometric combustion mode where a fuel-air mixture is introduced into a combustion chamber for performing a combustion, and a stratified charge combustion mode where a fuel is directly injected into the combustion chamber at a later stage of an air compression stroke. In the stoichiometric-combustion operation of this type of engine, the engine is likely to generate a vibration whose amplitude is relatively large, while the negative pressure in the air intake system of the engine, as a vacuum source, is decreased (close to the atmospheric pressure). In the stratified charge combustion operation of the engine, on the other hand, the engine is likely to generate a vibration whose amplitude is relatively small, while the negative pressure in the air intake system is increased. This characteristics of the proposed internal combustion engine causes a difficulty in tuning of the vibration damping device which is installed in the vehicle having the above-indicated improved internal combustion engine.
For instance, the vibration damping device which is tuned so as to exhibit a desired damping effect with respect to the vibration generated in the stoichiometric combustion operation of the engine, suffers from incapability of exhibiting a sufficient damping effect, i.e., generating a sufficient oscillating force with respect to the vibration generated in the stratified charge combustion operation of the engine. On the other hand, the vibration damping device which is tuned so as to exhibit a desired damping effect with respect to the vibration generated in the stratified charge combustion operation of the engine, is prone to generate the excessively large oscillating force with respect to the vibration generated in the stoichiometric combustion operation of the engine, possibly deteriorating the vibration in the vibratile member of the vibration system.